As is known in the art, one type of semiconductor device is high electron mobility transistor (HEMT). As is known in the art, such devices are typically used at microwave and millimeter frequencies to provide amplification of radio frequency signals. Typically, HEMTs are formed from Group III-V materials such as gallium arsenide (GaAs) or indium phosphide (InP). In a HEMT there is a doped donor/undoped spacer layer of one material and an undoped channel layer of a different material. A heterojunction is formed between the doped donor/undoped spacer layer and the undoped channel layer. Due to the conduction band discontinuity at the heterojunction, electrons are injected from the doped donor/undoped spacer layer into the undoped channel layer. Thus, electrons from the large bandgap donor layer are transferred into the narrow bandgap channel layer where they are confined to move only in a plane parallel to the heterojunction. Consequently, there is spatial separation between the donor atoms in the donor layer and the electrons in the channel layer resulting in low impurity scattering and good electron mobility. Thus, with a HEMT, a quantum well is formed between a large bandgap material (InAlAs in MHEMTs) and a small bandgap, high electron mobility channel (In0.60Ga0.40As) channel below it. Silicon, Si, doping is used to introduce electrons for conduction of current. The Si doping is insertion in the InAlAs at the interface with the InGaAs. The electrons all move in the InGaAs and the donors stay in the InAlAs. The electrons can thus achieve high mobility, because they travel in the InGaAs and avoid ionized-impurity scattering due to separation from the donor Si atoms.
One type of HEMT is a pseudomorphic HEMT (pHEMT). Such pHEMTs are formed on GaAs wafers, using strained InGaAs layers for the small bandgap channel layer with ˜18-20% indium.
As is also known in the art, metamorphic HEMTs (MHEMTs) are promising devices for solid-state power generation above 60 GHz. MHEMTs are formed on GaAs wafers that use a grading layer to transform the lattice constant from GaAs to near InP. Once this is done, strain-free layers of InGaAs with 53-60% indium can be grown. InAlAs forms the large bandgap layer. The gate electrode is in Schottky contact with this InAlAs layer. The MHEMTs are nearly identical to InP HEMTs except for the substrate. It should be noted that these devices also have a layer of InGaAs above the wideband layer, but it is used only for making ohmic contact. An opening is formed in this layer to put the gate electrode metal down, on the InAlAs layer, to form the Schottky contact.
MHEMTs offer considerably higher gain and efficiency as compared to established GaAs pHEMT technology at nearly the same cost. However, they suffer from surface states between the gate and drain which accumulate electrons and reduce the sheet electron density in the semiconductor channel between gate and drain. This decrease of electron density is passivation dependent and results in increased source and drain resistances, Rs and Rd along with a reduction of the peak drain current capability, Imax. The current reduction, called IV collapse, is most severe when the device's gate voltage is pulsed from −3V to 0V with greater than 0.5V on the drain. Additional reductions in Imax and device gain, Gm, occur gradually under high temperatures. Since the above conditions are encountered under RF power drive and bias, MHEMTs with standard silicon nitride passivation are expected to suffer poor reliability in power RF applications as measured by degradation in RF output power and gain.
Most attempts to solve this reliability problem involve filling an etched gate recess trench with gate metal. This technique attempts to reduce the effect of electron traps, at the semiconductor surface, by allowing the use of a thicker Schottky barrier layer that moves the semiconductor surface with its trapped electrons, farther from the HEMT channel. The resulting increased distance between the trapped electrons and HEMT channel reduces the ability of the trapped electrons to reduce the channel's electron density.
In order for the above method to work well, one must ensure that the gate metal covers the sidewalls of the gate recess trench so that no surface traps are left which would be close to the channel. This implies a minimum of undercut (ungated region) during the gate recess etch. Additionally, the gate metal must not contact the highly conductive cap layer since this would result in a short-circuit of the gate. Such requirements force one to fabricate a complicated double-recessed gate structure in which the gate length and/or ungated region is sensitive to gate etch time and hence difficult to control.